Achillobator giganticus

Perle, Norell, et Clark 1999

Evidence: partial skull and fragmentary skeleton

Cenomanian to Santonian

Bayanshiree Svita

Dornogov’; Mongolia

Biology: 5 meters long – 250 kilograms

Since it is known from only a single specimen, not much is known of Achillobator biology. It is believed that, given its comparably large size, it preyed on much larger dinosaurs than other dromaeosaurids might (Paul 2010). However, the most common herbivores of the Bayanshiree were ankylosaurs and therizinosaurs. Hadrosaurs were rare, with only one known species, Bactrosaurus mongoliensis. While it seems very likely that Achillobator might have preferred a hadrosaur to a potentially dangerous therizinosaur or ankylosaur but, given the frequencies of the later herbivores, it probably evolved its large size to cope with the more formidable herbivores. It probably wasn’t fast enough to take down the ornithomimid Garudimimus brevipes also found in the Bayanshiree. Achillobator may have competed with Alectrosaurus but, because of the dubious nature of that tyrannosaur, how they might have competed would be speculation at best. Perhaps the tyrannosaur pursued primarily hadrosaurs, limiting conflicts.

Evolution

There is very little controversy over the placement of Achillobator in the Dromaeosauridae and, more specifically, in the Dromaeosaurinae. It appears to be closest to Utahraptor and Dromaeosaurus (Senter et al.2012).

References:

Paul, G. S. 2010. The Princeton Field Guide to Dinosaurs. Princeton, NJ: Princeton University Press.

Senter, P., J. I. Kirkland, D. D. DeBlieux, S. Madsen, et N. Toth. 2012. “New Dromaeosaurids (Dinosauria: Theropoda) from the Lower Cretaceous of Utah, and the Evolution of the Dromaeosaurid Tail.” PLoS ONE 7(5): e36790.